1. Field of the Invention
The present invention relates to a method of fabricating a display device, and more particularly, to a method of fabricating a color filter substrate for a liquid crystal display device.
2. Discussion of the Related Art
With rapid development within the information technology field, display devices have evolved to be able to process and display increasingly large amounts of information. Flat panel display technologies recently have been developed for display devices having small thickness, light weight, and low power consumption. Among these technologies, liquid crystal display (LCD) devices commonly have been used in notebook computers and desktop computer monitors due to their superior image resolution, color image display, and image quality.
In general, an LCD device includes an upper substrate, a lower substrate, and a liquid crystal layer disposed between the upper and lower substrates. The LCD device makes use of optical anisotropy of liquid crystal material and produces images by varying light transmittance according to the alignment of liquid crystal molecules by an electric field.
The lower substrate, which is commonly referred to as an array substrate, includes thin film transistors and pixel electrodes, and is fabricated using repeated photolithographic processes to pattern thin films. The upper substrate, which is commonly referred to as a color filter substrate, includes a color filter layer for displaying color images. The color filter layer commonly includes sub-color filters of red (R), green (G), and blue (B), and is formed by various methods including, for example, a dyeing method, an electro-deposition method, a pigment dispersion method, and a printing method. In general, the pigment dispersion method is more commonly used because it forms a fine pattern with good reproducibility.
FIGS. 1A to 1D are cross sectional views of a method of fabricating a color filter substrate for a liquid crystal display (LCD) device according to the related art. Here, the pigment dispersion method is used.
In FIG. 1A, a black matrix 15 is formed on an insulating substrate 10 by depositing a metal material or coating a resin, and patterning the metal material or the resin through a photolithographic processes. The black matrix 15 blocks light leakage, which is caused by irregular operation of liquid crystal molecules, within regions except for pixel electrodes of an array substrate (not shown). The black matrix 15 also prevents light from being transmitted into a channel of a thin film transistor of the array substrate.
In FIG. 1B, a color resist 17, which may be one of red, green, and blue resists, for example a red one, is coated onto the substrate 10 including the black matrix thereon by spin coating. A mask 20 having a light transmitting portion and a light blocking portion is disposed over the red resist 17, and the red resist 17 is exposed to light using the mask 20. Here, the red resist 17 is shown to have a negative property, i.e., a portion of the red resist 17 that is not exposed to light is removed.
In FIG. 1C, the red resist 17 (in FIG. 1B) is developed, and a red color filter pattern 17a is formed. Then, the red color filter pattern 17a is cured and hardened.
In FIG. 1D, green and blue color filter patterns 17b and 17c are formed on the black matrix 15 through similar processes, as shown in FIGS. 1B and 1C. Next, an overcoat layer 23 and a common electrode 25 are subsequently formed on the substrate 10 including the color filter patterns 17a, 17b, and 17c. The overcoat layer 23 protects the color filter patterns 17a, 17b, and 17c, and flattens the surface of the substrate 10 having the color filter patterns 17a, 17b, and 17c. The common electrode 25 is made of a transparent conductive material, such as indium-tin-oxide and indium-zinc-oxide.
During the fabrication method of the color filter substrate using the pigment dispersion, since the color filter substrate is fabricated by repeated processes of coating, exposing, developing, and curing of a color resist, the fabrication method is complicated and requires significant amounts of time and numerous pieces of equipment. To solve the above problem, a fabrication method of a color filter substrate using thermal imaging has been proposed, as disclosed for example in U.S. Pat. No. 6,242,140, which is hereby incorporated by reference.
FIGS. 2A to 2D are cross sectional views of another method of fabricating a color filter substrate using thermal imaging according to the related art. In FIG. 2A, a black matrix 35 is formed on an insulating substrate 30 by depositing a metal material or coating a resin, and patterning the metal material or the resin by photolithographic processes.
In FIG. 2B, a first color transcription film 40 is disposed over the substrate 30 including the black matrix 35. The first color transcription film 40 includes a supporting film 40a, a light-to-heat conversion (LTHC) layer 40b, and a color filter layer 40c. 
In FIG. 2C, the first color transcription film 40 is adhered to the substrate 30 without bubbles. A laser head 50, from which a laser beam is generated, is disposed over the first color transcription film 40. Then, the laser beam is applied to the first color transcription film 40 within a portion where a first color filter pattern will be formed later while the laser head 50 is reciprocated along a straight line. In the first color transcription film 40 exposed to the laser beam, the LTHC layer 40b transforms light absorbed from the laser beam into thermal energy and emits the thermal energy. Accordingly, the color filter layer 40c is transferred onto the substrate 30 due to the emitted thermal energy. Here, the color filter substrate may be a stripe type where color filter patterns are disposed along a line each having the same color. Thus, a first line is exposed to the laser beam by moving the laser head along a straight line, but second and third lines are skipped. Similarly, a fourth line is exposed to the laser beam. Using these processes, all the lines of the first color filter pattern are exposed, and the first color transcription film 40 is removed.
In FIG. 2D, the first color filter pattern 45a is formed between the adjacent black matrixes 35 on the substrate 30, wherein the first color filter pattern 45a may be a red color filter. A second color filter pattern 45b and a third color filter pattern 45c are formed through the same process, as shown in FIGS. 2B and 2C, wherein the second and third color filer patterns 45b and 45c may be green and blue color filters, respectively. The substrate 30 having the color filter patterns 45a, 45b, and 45c is placed in a hardening furnace, and the color filter patterns 45a, 45b, and 45c are hardened. An overcoat layer 47 is formed on the color filter patterns 45a, 45b, and 45c in order to protect the color filter patterns 45a, 45b, and 45c, and to flatten the surface of the substrate 30 otherwise having steps. Next, a common electrode 49 is formed on the overcoat layer 47 by depositing a transparent conductive material, such as indium-tin-oxide and/or indium-zinc-oxide.
During the thermal imaging method, manufacturing throughput of the color filter substrate is influenced by an application direction of the laser beam, wherein the laser beam is applied to the transcription film along a direction parallel to a pixel length of the LCD device. For example, in a color filter substrate of a video graphic array (VGA) LCD device, which has a resolution of 640 by 480, the VGA LCD device has sub-pixels of 640 by 3 lines (i.e., 1920 lines). Thus, the laser head 50 must scan 640 times for each color filter pattern, and a total number of scans is 1920. In addition, a size of the pixel depends on the resolution being used (e.g., VGA, SVGA (super video graphic array), XGA (extended graphic array), and so on), thereby making it problematic to have a laser beam fit for each different pixel size.
The scanning of the laser head 50 may be accomplished along a direction parallel to a pixel width of the LCD device, thereby reducing the scanning times. This may be referred to as a horizontal laser scan. The manufacturing throughput of the color filter substrate is improved due to reduction of the scanning times. However, in this case, there is a problem that scanning traces may be formed at pixel regions, thereby reducing image quality.
FIG. 3 is a plan view of a color filter substrate fabricated by a thermal imaging method using a horizontal laser scan according to the related art. In FIG. 3, a substrate 30 includes a black matrix 35 and a color filter pattern 45 thereon, wherein the black matrix 35 has an opening in which the color filter pattern 45 is placed. The color filter pattern 45 is formed by the above-described thermal imaging method using a horizontal laser scan. A laser head 50 having a plurality of laser pixels 52 first scans the substrate 30 along a horizontal direction of the substrate 30 repeatedly turning the laser pixel 52 ON and OFF. After the first scan, the laser head 50 is moved along the vertical direction of the substrate 30 by a width of the first scan, and a second scan is accomplished. Here, a scanning trace 55 is formed along a border between first and second scanning regions, and is situated on the color filter pattern 45.
FIG. 4A is an enlarged view of a region A in FIG. 3 according to the related art, and FIG. 4B is a cross sectional view along IV—IV of FIG. 4A according to the related art.
In FIGS. 4A and 4B, after repeated laser scans, the scanning trace 55 is formed on the color filter pattern 45 because of scanning borders of the first and second scans. When a laser beam is applied to a light-to-heat conversion (LTHC) layer of a transcription film, photo energy from the laser beam applied to the LTHC layer is transformed into thermal energy. Accordingly, a color filter layer is transferred onto the substrate due to the thermal energy, wherein the color filter layer is actually transferred onto a larger area than the region actually exposed to the laser beam. In addition, due to difference in scanning times, spontaneous hardening of the color filter film, and the expansion rate of the color filter layer, the scanning trace 55 may have a certain thickness that protrudes over the surface of the color filter pattern 45. Thus, the scanning trace 55 on the color filter pattern 45 lowers image quality.